The present invention relates to a multi-speed transmission for transmitting power between a pair of shafts. In particular, the present invention relates to a multi-speed transmission wherein a uni-directional free wheeling mechanism is provided between a gear and its shaft so that the gear rotates freely about its shaft when the gear rotates in a preselected direction relative to the rotation of its shaft and so that the gear is locked to its shaft at all other times. As a result, only one gear set transmits power from an input shaft to an output shaft when a plurality of gears of the input shaft are interconnected to a plurality of gears of the output shaft.
Transmissions are utilized in a wide variety of vehicles and stationary equipment to transmit power and to change rotational speeds between input and output shafts. Generally, power is supplied to the input shaft either mechanically or non-mechanically. In applications where power is mechanically applied to the input shaft, a clutch mechanism is usually required to interrupt the transmission of power and to shift gear speeds.
Bicycles are examples where the power supply is provided to the input shaft by non-mechanical means. As a result, the rotation of the input shaft may be easily interrupted without a clutch mechanism. Bicycles typically include chain derailleur mechanisms for shifting a drive chain from one sprocket to an adjacent sprocket to change speed ratios. Most multi-speed bicycles have derailleurs and multiple sprockets at both the front pedal hubs and rear wheel hubs to provide a variety of speed ratios. Furthermore, most chain sprocket systems may be shifted under full power.
However, chain and sprocket systems suffer from several key disadvantages. The multiple sprockets at both the front pedal hubs and the rear wheel hubs are interconnected by the drive chain which extends along a substantial length of the bicycle. As a result, chain and sprocket systems are difficult to house or enclose. Consequently, the chain and sprockets are susceptible to damaging impacts and are exposed to road elements such as dirt and water. Damaging impacts may cause the derailleur mechanisms or sprockets to become bent or misaligned which may impair shifting between sprockets. Moreover, road elements may also come in contact with the derailleur, chain or sprocket to hamper shifting or even damage the components. As a result, the performance of chain and sprocket systems becomes impaired unless the chain and sprocket systems are periodically cleaned and adjusted.
In addition, chain and sprocket systems have inherent design limitations which limit the effective number of speed ratios which may be provided. As a result, chain and sprocket systems do not provide as many speed ratios as indicated by their conventional designations. For example, a bicycle having three sprockets at the front pedal hub and seven sprockets at the rear wheel hub is conventionally designated as a "21 speed" bicycle. Although the bicycle has 21 sprocket combinations, the "21 speed" bicycle actually provides only 11 speed ratios as illustrated by Table 1. Table 1 shows twenty-one sprocket combinations and their associated speed ratios for a typical "21-speed" bicycle. Table 1 also shows the percent of change between speed ratios of adjacent sprocket combinations.
TABLE 1 ______________________________________ Speed Ratios and Percent of Change of "21-Sprocket Combination" Chain and Sprocket System Pedal Hub Sprocket Teeth 48 38 28 Rear Percent Rear Percent Rear Percent Hub of Hub of Hub of Teeth Ratio Change Teeth Ratio Change Teeth Ratio Change ______________________________________ 30 3.69 15.4 26 3.20 13.3 23 2.82 30 2.92 17.6 15.4 20 2.40 26 2.53 15.0 13.3 17 2.09 23 2.24 30 2.15 13.0 17.6 15.4 15 1.85 20 1.90 26 1.87 15.4 15.0 13.3 13 1.60 17 1.65 23 1.65 13.0 17.6 15 1.46 20 1.40 15.4 15.0 13 1.27 17 1.22 13.0 15 1.08 15.4 13 0.93 ______________________________________
As shown in bold by Table 1, the speed ratios provided by the second hub sprocket having 38 teeth and the third hub sprocket having 28 teeth are redundant except for the two lower speed ratios of each sprocket. Similarly, an 18 speed system provides only 10 effective ratios, a 14-speed system provides only 9 effective ratios and a 12-speed system provides only 8 effective ratios.
As further illustrated by Table 1, chain and sprocket systems also fail to provide a smooth power curve. The number of teeth in the rear hub sprockets of chain and sprocket systems are typically selected so as to provide a degree of uniformity in speed ratios when used in conjunction with the pedal hub sprockets. However, because the number of teeth and the size of sprockets used on bicycles is limited, the combinations of front and rear sprocket teeth are also limited. As a result, chain and sprocket systems result in an erratic percent of change between speed ratios which results in a poor power curve. A preferred power curve for most bicycle applications would have a lower percent of change between lower speed ratios and a higher "overdrive" percent of change between higher speed ratios.
Furthermore, chain and sprocket systems require extensive shifting of both the front and rear sprockets. For example, shifting of the front sprocket may require additional shifting of the rear sprocket by several ranges just to maintain the same speed ratio. In addition, chain and sprocket systems also require shifting twice or "cross-shifting" to reach all speed ratios. As illustrated in Table 1 for the 21-sprocket combination bicycle, shifting from the highest speed ratio of the pedal hub sprocket with 28 teeth to the next higher speed ratio would require first shifting to the pedal hub sprocket with 38 teeth and then shifting from the rear hub sprocket with 30 teeth to the rear hub sprocket with 26 teeth. This extensive shifting is difficult as well as confusing.
In an attempt to protect chain and sprocket systems from road elements such as dirt and water, several modifications have been made to multi-speed bicycle transmissions. For example, Hartman U.S. Pat. No. 4,770,433 (Hartman '433) discloses an enclosed multiple speed drive for mountain bicycles. Hartman '433 discloses four drive gears rotatably mounted on a drive carrier and three drive gears rotatably mounted on a driven carrier. Each gear is meshed with a corresponding gear on each of four countershafts. Hartman '433 utilizes a first gear selector to lock a selected drive gear to the drive gear carrier and a second gear selector to lock a selected driven gear to the driven gear carrier. The shifting arrangement of Hartman '433 requires that each gear selector be moved to a neutral position before engaging a successive gear. As a result, the transmission does not provide continuous power while shifting. In addition, because two shifting mechanisms are employed, "cross-shifting" is required to reach all possible speed ratios. Furthermore, the selection of speed ratios is also limited because the distance from the center line of the gear carrier to the center line of the countershaft is fixed and the sum of the radii of each gear set must exactly match this preset distance. The free-wheeling mechanism shown in FIG. 4 of Hartman '433 is external to the transmission and allows the rear wheel to continue to rotate if the rider ceases pedalling. Because all thirty-five gears of Hartman '433 are meshed and rotating continuously, the transmission of Hartman creates considerable friction and results in wasted effort on the part of the rider.
Similarly, Bailey U.S. Pat. No. 4,926,714 (Bailey '714) describes an enclosed multi-gear transmission that requires reverse pedalling to shift gears. However, as with Hartman '433, the transmission of Bailey '714 does not allow for shifting under continuous power. Bailey '714 discloses gears rotatably positioned about the pedal shaft. Each gear includes an internal pawl disposed in a pawl slot within a pedal shaft Bailey '714 changes or shifts gear speeds by axially positioning a single pawl selector finger within a channel adjacent the slot within the pedal shaft to cause a selected pawl of a selected gear to engage the selected gear so as to couple the shaft to the selected gear. Movement of the pawl selector finger out from under the selected pawl causes the selected pawl to retract within the slot and to be completely disengaged from its corresponding gear. By selectively positioning the pawl selector finger, different pawls may be forced into engagement with their respective gears to provide different speed ratios. As the pawl selector finger 108 is axially moved between pawls, power from the pedal shaft is not transmitted to the output shaft via any gears. The shifting arrangement of Bailey '714 does not provide continuous power while shifting. Because the individual pawls of each gear are not biased so as to be in constant engagement with each pawl's respective gear, the pawls of Bailey '714 do not automatically lock the input shaft to the gear in response to the relative rotational directions of the pedal shaft and the gear.
Moreover, because the center line of the gear shafts are a fixed dimension apart, the transmission of Bailey '714 also provides only a restricted number of different gear radii and speed ratios. As in Hartman '433, the transmission of Bailey '714 requires that all of the gears mesh with one another and rotate continuously. Consequently, a considerable amount of friction and wasted effort results.